Typically, a horizontal axis wind turbine comprises a rotor to which one, two or more blades are attached radially from a hub, a nacelle which rotatably supports the rotor by means of supporting a main shaft of the rotor which extends in the horizon and to which the hub is fixed, and a tower that supports the nacelle such that there is a free yaw rotation.
As a typical yaw rotation drive mechanism there is a drive mechanism in which the top section of the tower and the nacelle are linked by a yaw bearing having a yaw axis as the axis of rotation, an inner-teeth ring gear is attached to the inner race that is fastened on the tower side of the yaw bearing, a yaw motor is located on the nacelle, a pinion gear that is fastened to the drive shaft of the yaw motor engages with the ring gear, and by driving and rotating the yaw motor, the nacelle is rotated around the yaw axis. With this mechanism, the yaw angle of the nacelle is controlled so that the nacelle, and thus the rotor faces in a desired direction such as the direction of the incoming wind (for example, refer to Japanese Patent Publication No. 2001-289149).
Not only does the wind speed change with respect to the wind turbine, but also turbulence such as sudden crosswinds, blowing upward from underneath, or blowing downward from above occurs. These have a large effect on the operation of the wind turbine.
When turbulent wind occurs, swinging motion of the nacelle occurs due to backlash in the ring gear on the tower side and yaw gear on the nacelle side. Swinging motion of the nacelle could be a cause of damage to the yaw gear, yaw motor, rotor and the like.